July 2021
July 2021 - Medieval flatfish and what we can learn from them about the origins of European sea fishing
Katrien Dierickx (ESR 4 - York)
Flatfish are some of the weirdest fish you can find. Their eyes, both situated on the same side of their head, can protrude from under the sand on the bottom of the sea or river estuaries, where these fish lie in waiting for prey, or seemingly stare at you from the fishmongers’ stall (fig. 1).
Flatfish are a common group of fish people eat nowadays. Turbot, plaice, Dover sole, and halibut are practically permanent residents on western European market shelves. This was no different a thousand years ago. Or was it? I will try to answer this question as part of my research project on medieval flatfish from the North Sea.
Fig. 1. A typical modern western European fish stall with many different flatfish species.
Fig. 2. Medieval flatfish remains from the Barreau Saint-George site in Northern France.
The oldest records of human consumption of flatfish in Europe date from 10,000 years ago1,2. They seem to become much more frequent in archaeological finds during the Early and High Medieval Period (10-11th century C.E) in England and Flanders, around the same time as the Fish Event Horizon3,4,5,6,7. This event is characterized by a sudden increase in the consumption of marine fish such as codfishes, whereas freshwater species seem to become less prominent in people's diet3.
So the questions we can ask: what happened to the flatfish fisheries during the medieval period? Was there a shift from more freshwater and estuarine to more marine flatfish exploitation? Was there a shift towards more intense fishing of certain species, or were the same species targeted before, during, and after the Fish Event Horizon? If we want to answer these questions, we have to dig deeper into the archaeological flatfish remains (fig. 2).
The main issue that has limited research regarding these questions is the lack of good identification criteria for distinguishing between the most common flatfish species from the archaeological record around the North Sea. Although previous researchers were able to identify samples in some cases (mostly thanks to an in-depth comparison of the bones of three species8), only 1-15% of all flatfish bones recovered are currently identified to species5,7,9,10,11,12.
To increase the success rate of flatfish identifications, we are expanding the analysis by Wouters et al. (2007) using modern museum specimens to include other skeletal elements and several other species that occur in the North Sea in order to find diagnostic morphological characteristics of the bones that can be used to easily identify bones on sight (fig. 3).
Fig. 3. Comparison of modern flatfish bones to find diagnostic criteria. A. A view of relevant materials; B. Dentaries, or lower jaw bones, of two different flatfish species from museum collections.
Fig. 4. Geometric morphometric analysis of flatfish bones using landmarks. A. Photography of modern samples at home during lockdown; B. Example of some landmarks of a vertebra of a flatfish; C. Plot showing how taxa (color coded) can be differentiated by shape.
Unfortunately, we are not always lucky enough to find the bones of the whole fish and it is often just the vertebrae that survive. These are notorious for being difficult to identify on sight, because of the lack of diagnostic criteria. Nowadays, there are many simple identification tools and apps that simply require a photo to tell you what an object is. So, why not try something similar for flatfish vertebrae?
Geometric morphometrics is a shape analysis technique which uses a coordinate system of specific points on photos, called landmarks, to calculate how similar shapes are to each other13,14. By taking photographs of modern flatfish vertebrae from museum collections, we can create a reference set that can then be used to see to which species an archaeological sample is the most similar15 (fig. 4).
Unfortunately, bones that had been thrown away by someone a thousand years ago and have lain beneath the ground ever since, are not always complete enough to identify visually, nor by geometric morphometric analysis. This means that many bones often go unidentified… unless we use a fancy modern molecular technique. Enter: ZooMS, or Zooarchaeology by Mass Spectrometry.
This technique uses collagen, an abundant protein in bone tissue that preserves relatively well for thousands of years16,17,18. By simply dissolving the mineral component of bone using acids (you can try this at home with some chicken bones in vinegar), the bone becomes squishy and can be further treated to extract the collagen. Tiny amounts of processed and cut-up collagen can then be analysed using a mass spectrometer, which separates different parts of the collagen based on their mass.
After the collagen is processed in the mass spectrometer, you can see a spectrum showing different masses associated with different cut-up parts of the collagen. Each flatfish species has a slightly different constitution of collagen, and therefore will show a different spectrum or ‘fingerprint’. By comparing the spectrum of an archaeological sample with a reference library, species can often be identified easily19,20,21,22 (fig. 5).
Fig. 5. ZooMS analysis of an archaeological sample. A. Bones undergoing demineralisation in eppendorf tubes with acid; B. Collagen extractions plated and ready for analysis in a mass spectrometer; C. Spectrum showing the differences between two species.
Species identification of flatfish is not enough to answer all our questions about medieval fisheries. Flatfish are generally marine species. However, juveniles of several species and adults of some species can occur in estuarine and freshwater systems23,24. There is no clear distinction between freshwater-caught or marine-caught flatfish using only species identification. To solve this problem, we have to use another molecular technique: stable isotope analysis.
Fig. 6. Stable isotope analysis of archaeological samples. A. Bone undergoing demineralisation with acid; B. Collagen extractions of two bones; C. Placing 0.4-0.6 mg of extracted collagen of a sample into a tiny capsule for the mass spectrometer.
Isotopes of a chemical element differ in their number of neutrons, which makes them heavier or lighter and quicker or slower to be incorporated into an organism’s body than other isotopes of the same element. This leads to different isotope proportions in organisms based on what they eat and where they live. Carbon and nitrogen isotopes preserved in collagen can provide signatures typical of either freshwater or marine habitats25,26.
Tiny amounts of extracted bone collagen from an archaeological sample (the same stuff used for ZooMS), are analysed with a mass spectrometer, which determines the relative proportions of isotopes in a sample (fig. 6). Using this technique, the catch habitat of medieval flatfish can be analysed27,28.
The combination of these different techniques is applied to analyse and compare archaeological sites from around the North Sea, from England, France, Belgium and the Netherlands. Hopefully, the results can tell us much more about flatfish fisheries during the medieval period and what changed during the Fish Event Horizon. Not only will this potentially tell us more about how humans lived and fished during the past, it might also provide us with some more insight into modern day flatfish fisheries and how we can make sure that there will still be enough flatfish in the North Sea in the future.
References
(1) Adán, G.E., Alvarez-Lao, D., Turrero, P., Arbizu, M., Garciá-Vázquez, E., 2009. Fish as diet resource in North Spain during the Upper Paleolithic. Journal of Archaeological Science 36: 895-899.
(2) Turrero, P., Ardura, A., Garciá-Vázquez, E., 2014. Paleolithic vs. Epipaleolithic fisheries in northern Iberia. Quaternary Research 82: 51-55.
(3) Barrett, J.H., Locker, A.M., Roberts, C.M., 2004. The origins of intensive marine fishing in medieval Europe: the English evidence. Proc. R. Soc. Lond. 271: 2417-2421.
(4) Van Neer, W., Ervynck, A., 2007. De zoöarcheologische studie van de ontwikkeling van de exploitatie van de zee: een status quaestionis voor Vlaanderen, in: de Kraker, A.M.J. et al. (Ed.) Veen-Vis-Zout. Landschappelijke dynamiek in de zuidwestelijke delta van de Lage Landen. Geoarchaeological and Bioarchaeological Studies, 8: pp. 45-54.
(5) Harland, J.F., Jones, A.K.G., Orton, D.C., Barrett, J.H., 2016. Fishing and fish trade in medieval York: The zooarchaeological evidence: 172-204. In Barrett, J.H., Orton, D.C. (Eds.). Cod and herring: The archaeology and history of medieval sea fishing. Oxford, England: Oxbow Books.
(6) Van Neer, W., Ervynck, A., 2016. The rise of sea-fish consumption in inland Flanders, Belgium. In: Cod and Herring: The Archaeology and History of Medieval Sea Fishing, ed. by Barrett J.H & Orton D.C.. Oxbow Books, chap. 14, pp. 156-171.
(7) Oueslati, T., 2019. A French fish event at the turn of the 10th century? Environment, economy, and ethnicity in maritime Flanders. International Journal of Osteoarchaeology 29: 443-451.
(8) Wouters W., Muylaert L., Van Neer W., 2007. The distinction of isolated bones from plaice (Pleuronectes platessa), flounder (Platichthys flesus) and dab (Limanda limanda): a description of the diagnostic characters. Archaeofauna 16: 33-72.
(9) Ervynck, A., Van Neer, W., 1992. De voedselvoorziening in de Sint-Salvatorsabdij te Ename (stad Oudenaarde, prov. Oost-Vlaanderen) I. Beenderen onder een keukenvloer (1450-1550A.D.). Archeologie in Vlaanderen 2: 419-434.
(10) Bødker Enghoff, I., 1999. Fishing in the Baltic Region from the 5th century BC to the 16th century AD: evidence from fish bones. Archaeofauna 8: 41-85.
(11) Nicholson, R.A., 2009. Southampton French Quarter 1382 Specialist Report Download E2: Fish Bone (Specialist Report Download E2). Oxford Archaeology, Oxford. http://library.thehumanjourney.net/58/1/SOU_1382_Specialist_report_download_E2.pdf
(12) Reynolds, R.V., 2015. Food for the Soul: The Dynamics of Fishing and Fish Consumption in Anglo-Saxon England: c. A.D. 410-1066. PhD thesis, University of Nottingham: 403p.
(13) Bookstein, F.L., 1991. Morphometric tools for landmark data. Geometry and Biology. Cambridge University Press: 435p.
(14) Zelditch, M.L., Swiderski, D.L., Sheets, H.D., 2012. Geometric morphometrics for biologists. 2nd edition. Elsevier Science & Technology: 489p.
(15) Guillaud, E., Cornette, R., Bearez, P., 2016. Is vertebral form a valid species-specific indicator for salmonids? The discrimination rate of trout and Atlantic salmon from archaeological to modern times. Journal of Archaeological Science 65: 84-92.
(16) Collins, M., Harland, J., Buckley, M., Jones, A., 2009. Zoo-MS: zooarchaeology by Mass spectrometry, collagen as a molecular fingerprint for fish remains? Environment and Culture 7: 191-193.
(17) Buckley, M., Collins, M., Thomas-Oates, J., Wilson, J. C., 2009. Species identification by analysis of bone collagen using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. Rapid Communications in Mass Spectrometry 23: 3843–3854.
(18) Collins, M., Buckley, M., Grundy, H.H., Thomas-Oates, J., Wilson, J., van Doorn, N., 2010. ZooMS: the collagen barcode and fingerprints. Spectroscopy 22: 6-10.
(19) Korzow Richter, K., Wilson, J., Jones, A.K.G., Buckley, M., van Doorn, N., Collins, M.J., 2011. Fish ‘n chips: ZooMS peptide mass fingerprinting in a 96 well plate format to identify fish bones fragments. Journal of Archaeological Science 38: 1502-1510.
(20) Harvey, V.L., Daugnora, L., Buckley, M., 2018. Species identification of ancient Lithuanian fish remains using collagen fingerprinting. Journal of Archaeological Science 98: 102-111.
(21) Rick, T., Harvey, V.L., Buckley, M., 2019. Collagen fingerprinting and the Chumash billfish fishery, Santa Barbara Channel, California, USA. Archaeological and Anthropological Sciences 11: 6639-6648.
(22) Korzow Richter, K., McGrath, K., Masson-MacLean, E., Hickinbotham, S., Tedder, A., Britton, K., Bottomley, Z., Dobney, K., Hulme-Beaman, A., Zona, M., Fischer, R., Collins, M.J., 2020. What’s the catch? Archaeological application of rapid collagen-based species identification for Pacific Salmon. Journal of Archaeological Science 116: 105116.
(23) Gibson, R.N., Nash, R., Geffen, A.J., van der Veer, H.W., 2015. Flatfish: biology and exploitation. Second edition. John Wiley and sons: 567p.
(24) McGoran, A.R., Morritt, C.D., 2017. Presence of microplastics in the digestive tracts of European flounder, Platichthys flesus, and European smelt, Osmerus eperlanus, from the River Thames. Environmental Pollution 220: 744-751.
(25) Hobson, K.A., Clark, R.G., 1992. Assessing avian diets using stable isotopes II: factors influencing diet-tissue fractionation. The Condor 94: 189-197.
(26) Katzenberg, M.A., 2008. Stable isotope analysis: a tool for studying past diet, demography, and life history: 413-442. In: Katzenberg, M.A., Saunders, S.R. (eds). Biological Anthropology of the Human Skeleton. 2nd Edition. Hoboken, NJ : Wiley.
(27) Fuller, B.T., Muldner, G., Van Neer, W., Ervynck, A., Richards, M.P., 2012. Carbon and nitrogen stable isotope ratio analysis of freshwater, brackish and marine fish from Belgian archaeological sites (1st and 2nd millennium AD). Journal of Analytical Atomic Spectrometry 27: 807-820.
(28) Robson, H.K., Andersen, S.H., Clarke, L., Craig, O.E., Gron, K.J., Jones, A.K.G., Karsten, P., Milner, N., Price, T.D., Ritchie, K., Zabilska-Kunek, M., Heron, C., 2016. Carbon and nitrogen stable isotope values in freshwater, brackish and marine fish bone collagen from Mesolithic and Neolithic sites in central and northern Europe. Environmental Archaeology 21: 105-118.